I just went through it, it is actually difficult, but not THAT difficult. You don't flip a flag, you implement a Command pattern of sorts: instead of deleting a

I just went through it, it is actually difficult, but not THAT difficult. You don't flip a flag, you implement a Command pattern of sorts: instead of deleting an object you record the command "DELETE" along with the copy of the object. When you want to undo delete, you take this record, change "DELETE" to "CREATE" and submit this command to execution. Then all the constraint logic works as if you're creating the object from scratch, with possible conflict resolution along the way. Undo is easier if it's not an implementation of time travel but redoing something you did previously in the opposite direction.